Real-Time High-Sensitivity Reaction Monitoring of Important Nitrogen-Cycle Synthons by ¹⁵N Hyperpolarized Nuclear Magnetic Resonance

Here, we show how signal amplification by reversible exchange hyperpolarization of a range of 15N-containing synthons can be used to enable studies of their reactivity by 15N nuclear magnetic resonance (NO2- (28% polarization), ND3 (3%), PhCH2NH2 (5%), NaN3 (3%), and NO3- (0.1%)). A range of iridium-based spin-polarization transfer catalysts are used, which for NO2- work optimally as an amino-derived carbene-containing complex with a DMAP-d2 coligand. We harness long 15N spin-order lifetimes to probe in situ reactivity out to 3 × T1. In the case of NO2- (T1 17.7 s at 9.4 T), we monitor PhNH2 diazotization in acidic solution. The resulting diazonium salt (15N-T1 38 s) forms within 30 s, and its subsequent reaction with NaN3 leads to the detection of hyperpolarized PhN3 (T1 192 s) in a second step via the formation of an identified cyclic pentazole intermediate. The role of PhN3 and NaN3 in copper-free click chemistry is exemplified for hyperpolarized triazole (T1 < 10 s) formation when they react with a strained alkyne. We also demonstrate simple routes to hyperpolarized N2 in addition to showing how utilization of 15N-polarized PhCH2NH2 enables the probing of amidation, sulfonamidation, and imine formation. Hyperpolarized ND3 is used to probe imine and ND4+ (T1 33.6 s) formation. Furthermore, for NO2-, we also demonstrate how the 15N-magnetic resonance imaging monitoring of biphasic catalysis confirms the successful preparation of an aqueous bolus of hyperpolarized 15NO2- in seconds with 8% polarization. Hence, we create a versatile tool to probe organic transformations that has significant relevance for the synthesis of future hyperpolarized pharmaceuticals.